1. Field of the Invention
The instant disclosure relates to a RF power amplifier; in particular, to a RF amplifier which can self-adjust a bias point.
2. Description of Related Art
With the Internet well developed, people are used to receiving mass information fast, especially because the wireless communication technology has developed in recent years and many kinds of personal mobile products are blooming such as the mobile phone, the Personal Digital Assistance (PDA) and the like, people tend to have the real-time on-line support in addition to capturing the real-time information. Therefore, the combination of the Internet, the Wireless Local Area Network (WLAN) regarding to the wireless communication and the 3rd-Generation (4G) and 4th-Generation (4G) mobile communication technologies becomes needed.
A RF power amplifier plays an extremely important role in the communication system. In order to have better Power Added Efficiency (PAE), a linear RF power amplifier usually prefers to the class AB operation. Generally speaking, when a RF input power increases, an average voltage drop of the forward biased junction of an amplification transistor decreases and the RF power amplifier is forced to the class B operation and even the class C operation, so that the output power of the amplification transistor becomes saturated and the output signal gradually becomes non-linear.
In other words, in a transmitter, the RF power amplifier is an element having the most power consumption, of which the Power Added Efficiency (PAE) directly affects the battery life. A direct current bias of the class AB can improve the Power Added Efficiency (PAE) of the power amplifier, because it has a lower level quiescent current, which is, the bias current with zero input power. However, when the input power of the amplifier increases to certain level, the load line used in the class AB amplifier may enter into a cut-off region, which further results in a gain compression making the output power saturated. When the input power of the class AB amplifier increases to certain level, the gain compression mechanism in the cut-off region restricts the power gain and the output power of the class AB amplifier, because the direct current operating point is near the cut-off region.